What makes a nuclear reactor meltdown

While measures can be taken to limit human uptake of I, evacuation of area for several weeks, iodide tablets , high levels of radioactive caesium can preclude food production from affected land for a long time. Other radioactive materials in a reactor core have been shown to be less of a problem because they are either not volatile strontium, transuranic elements or not biologically active tellurium, xenon Accidents in any field of technology provide valuable knowledge enabling incremental improvement in safety beyond the original engineering.

Cars and airliners are the most obvious examples of this, but the chemical and oil industries can provide even stronger evidence. Civil nuclear power has greatly improved its safety in both engineering and operation over its 65 years of experience with very few accidents and major incidents to spur that improvement.

The Fukushima Daiichi accident was the first since TMI in which will have significant implications, at least for older plants. A scram is a sudden reactor shutdown. When a reactor is scrammed, automatically due to seismic activity, or due to some malfunction, or manually for whatever reason, the fission reaction generating the main heat stops.

However, considerable heat continues to be generated by the radioactive decay of the fission products in the fuel. Even then it must still be cooled, but simply being immersed in a lot of water does most of the job after some time. Aspects of nuclear plant safety highlighted by the Fukushima accident were assessed in the nuclear reactors in the EU's member states, as well as those in any neighbouring states that decided to take part. They were conducted from June to April It then negotiated the scope of the tests with the European Nuclear Safety Regulators Group ENSREG , an independent, authoritative expert body created in by the European Commission comprising senior officials from the national nuclear safety, radioactive waste safety or radiation protection regulatory authorities from all EU member states, and representatives of the European Commission.

In June the governments of seven non-EU countries agreed to conduct nuclear reactor stress tests using the EU model. Armenia, Belarus, Croatia, Russia, Switzerland, Turkey and Ukraine signed a declaration that they would conduct stress tests and agreed to peer reviews of the tests by outside experts. Russia had already undertaken extensive checks. The reassessment of safety margins is based on the existing safety studies and engineering judgement to evaluate the behaviour of a nuclear power plant when facing a set of challenging situations.

For a given plant, the reassessment reports on the most probable behaviour of the plant for each of the situations considered. The results of the reassessment were peer-reviewed and shared among regulators. WENRA noted that it remains a national responsibility to take or order any appropriate measures, such as additional technical or organisational safety provisions, resulting from the reassessment.

The scope of the assessment took into account the issues directly highlighted by the events in Fukushima and the possibility for combination of initiating events. Two 'initiating events' were covered in the scope: earthquake and flooding. The consequences of these — loss of electrical power and station blackout, loss of ultimate heat sink and the combination of both — were analysed, with the conclusions being applicable to other general emergency situations.

In accident scenarios, regulators consider power plants' means to protect against and manage loss of core cooling as well as cooling of used fuel in storage. They also study means to protect against and manage loss of containment integrity and core melting, including consequential effects such as hydrogen accumulation.

Nuclear plant operators start by documenting each power plant site. This analysis of 'extreme scenarios' followed what ENSREG called a progressive approach "in which protective measures are sequentially assumed to be defeated" from starting conditions which "represent the most unfavourable operational states. The documents had to cover provisions in the plant design basis for these events and the strength of the plant beyond its design basis. This means the "design margins, diversity, redundancy, structural protection and physical separation of the safety relevant systems, structures and components and the effectiveness of the defence-in-depth concept.

For severe accident management scenarios they must identify the time before fuel damage is unavoidable and the time before water begins boiling in used fuel ponds and before fuel damage occurs. Measures to prevent hydrogen explosions and fires are to be part of this. Since the licensee has the prime responsibility for safety, they performed the reassessments, and the regulatory bodies then independently reviewed them.

The exercise covered nuclear plants in 15 EU countries — including Lithuania with only decommissioned plants — plus 15 reactors in Ukraine and five in Switzerland. Operators reported to their regulators who then reported progress to the European Commission by the end of Information was shared among regulators throughout this process before the 17 final reports went to peer-review by teams comprising 80 experts appointed by ENSREG and the European Commission.

The final documents were published in line with national law and international obligations, subject only to not jeopardising security — an area where each country could behave differently.

The process was extended to June to allow more plant visits and to add more information on the potential effect of aircraft impacts. The full report and a summary of the 45 recommendations were published on www. The results of the stress tests pointed out, in particular, that European nuclear power plants offered a sufficient safety level to require no shutdown of any of them. At the same time, improvements were needed to enhance their robustness to extreme situations.

In France, for instance, they were imposed by ASN requirements, which took into account exchanges with its European counterparts. The EU process was completed at the end of September , with the EU Energy Commissioner announcing that the stress tests had showed that the safety of European power reactors was generally satisfactory, but making some other comments and projections which departed from ENSREG. The first order required the addition of equipment at all plants to help respond to the loss of all electrical power and the loss of the ultimate heat sink for cooling, as well as maintaining containment integrity.

Another required improved water level and temperature instrumentation on used fuel ponds. The third order applied only to the 33 BWRs with early containment designs, and required 'reliable hardened containment vents' which work under any circumstances.

In Japan similar stress tests were carried out in under the previous safety regulator, but then reactor restarts were delayed until the newly constituted Nuclear Regulatory Authority devised and published new safety guidelines, then applied them progressively through the fleet. Volcanic hazards are minimal for practically all nuclear plants, but the IAEA has developed a new Safety Guide on the matter. The Bataan plant in Philippines which has never operated, and the Armenian plant at Metsamor are two known to be in proximity to potential volcanic activity.

Nuclear plants are usually built close to water bodies, for the sake of cooling. The site licence takes account of worst case flooding scenarios as well as other possible natural disasters and, more recently, the possible effects of climate change. As a result, all the buildings with safety-related equipment are situated on high enough platforms so that they stand above submerged areas in case of flooding events. Occasionally in the past some buildings have been sited too low, so that they are vulnerable to flood or tidal and storm surge, so engineered countermeasures have been built.

EDF's Blayais nuclear plant in western France uses seawater for cooling and the plant itself is protected from storm surge by dykes. However, in a 2. For security reasons it was decided to shut down the three reactors then under power the fourth was already stopped in the course of normal maintenance. This incident was rated 2 on the INES scale. In the Kakrapar nuclear power plant near the west coast of India was flooded due to heavy rains together with failure of weir control for an adjoining water pond, inundating turbine building basement equipment.

The back-up diesel generators on site enabled core cooling using fire water, a backup to process water, since the offsite power supply failed. Following this, multiple flood barriers were provided at all entry points, inlet openings below design flood level were sealed and emergency operating procedures were updated. Construction of the Kalpakkam plant was just beginning, but the Madras plant shut down safely and maintained cooling.

However, recommendations including early warning system for tsunami and provision of additional cooling water sources for longer duration cooling were implemented. Three of the six reactors were operating at the time, and had shut down automatically due to the earthquake. The back-up diesel generators for those three units were then swamped by the tsunami. This cut power supply and led to weeks of drama and loss of the reactors.

The design basis tsunami height was 5. Tsunami heights coming ashore were about 14 metres for both plants. Unit 3 of Daini was undamaged and continued to cold shutdown status, but the other units suffered flooding to pump rooms where equipment transfers heat from the reactor circuit to the sea — the ultimate heat sink. The maximum amplitude of this tsunami was 23 metres at point of origin, about km from Fukushima. In the last century there had been eight tsunamis in the Japan region with maximum amplitudes above 10 metres some much more , these having arisen from earthquakes of magnitude 7.

Those in and in were the most recent affecting Japan, with maximum heights This earthquake was magnitude 9. For low-lying sites, civil engineering and other measures are normally taken to make nuclear plants resistant to flooding. Lessons from Blayais and Fukushima have fed into regulatory criteria.

However, few parts of the world have the same tsunami potential as Japan, and for the Atlantic and Mediterranean coasts of Europe the maximum amplitude is much less than Japan. In any light-water nuclear power reactor, hydrogen is formed by radiolytic decomposition of water. This needs to be dealt with to avoid the potential for explosion with oxygen present, and many reactors have been retrofitted with passive autocatalytic hydrogen recombiners in their containment, replacing external recombiners that needed to be connected and powered, isolated behind radiological barriers.

Also in some kinds of reactor, particularly early boiling water types, the containment is rendered inert by injection of nitrogen. As of early , a few in Spain and Japan did not have them.

Areva received in October a bulk order to supply its passive hydrogen recombiners to multiple Japanese units. This is beyond the capability of the normal hydrogen recombiners to deal with, and operators must rely on venting to atmosphere or inerting the containment with nitrogen.

There is a lot of international collaboration, but it has evolved from the bottom, and only in s has there been any real top-down initiative. In the aviation industry the Chicago Convention in the late s initiated an international approach which brought about a high degree of design collaboration between countries, and the rapid universal uptake of lessons from accidents. There are cultural and political reasons for this which mean that even the much higher international safety collaboration since the s is still less than in aviation.

International cooperation on nuclear safety issues takes place under the auspices of the World Association of Nuclear Operators WANO which was set up in In practical terms this is the most effective international means of achieving very high levels of safety through its four major programs: peer reviews; operating experience; technical support and exchange; and professional and technical development.

WANO peer reviews are the main proactive way of sharing experience and expertise, and by the end of every one of the world's commercial nuclear power plants had been peer-reviewed at least once. Following the Fukushima accident these have been stepped up to one every four years at each plant, with follow-up visits in between, and the scope extended from operational safety to include plant design upgrades.

Pre-startup reviews of new plants are being increased. Its aim is to legally commit participating States operating land-based nuclear power plants to maintain a high level of safety by setting international benchmarks to which States would subscribe. These obligations cover for instance, siting, design, construction, operation, the availability of adequate financial and human resources, the assessment and verification of safety, quality assurance and emergency preparedness.

The Convention is an incentive instrument. It is not designed to ensure fulfilment of obligations by Parties through control and sanction, but is based on their common interest to achieve higher levels of safety. These levels are defined by international benchmarks developed and promoted through regular meetings of the Parties.

The Convention obliges Parties to report on the implementation of their obligations for international peer review. This mechanism is the main innovative and dynamic element of the Convention. Under the Operational Safety Review Team OSART program dating from international teams of experts conduct in-depth reviews of operational safety performance at a nuclear power plant. They review emergency planning, safety culture, radiation protection, and other areas. The Convention entered into force in October As of August , there were 88 signatories to the Convention, 65 of which are contracting parties, including all countries with operating nuclear power plants.

The plan arose from intensive consultations with Member States but not with industry, and was described as both a rallying point and a blueprint for strengthening nuclear safety worldwide. It contains suggestions to make nuclear safety more robust and effective than before, without removing the responsibility from national bodies and governments.

It aims to ensure "adequate responses based on scientific knowledge and full transparency". Apart from strengthened and more frequent IAEA peer reviews including those of regulatory systems , most of the 12 recommended actions are to be undertaken by individual countries and are likely to be well in hand already.

Following this, an extraordinary general meeting of 64 of the CNS parties in September gave a strong push to international collaboration in improving safety. National reports at future three-yearly CNS review meetings will cover a list of specific design, operational and organizational issues stemming from Fukushima lessons. They include further design features to avoid long-term offsite contamination and enhancement of emergency preparedness and response measures, including better definition of national responsibilities and improved international cooperation.

Parties should also report on measures to "ensure the effective independence of the regulatory body from undue influence. However, in line with Swiss and EU intentions, "comprehensive and systematic safety assessments are to be carried out periodically and regularly for existing installations throughout their lifetime in order to identify safety improvements Reasonably practicable or achievable safety improvements are to be implemented in a timely manner.

An IAEA Design Safety Review DSR is performed at the request of a member state organization to evaluate the completeness and comprehensiveness of a reactor's safety documentation by an international team of senior experts. It is based on IAEA published safety requirements. Therefore, it is neither intended nor possible to cover or substitute licensing activity, or to constitute any kind of design certification.

In relation to Eastern Europe particularly, since the late s a major international program of assistance was carried out by the OECD, IAEA and Commission of the European Communities to bring early Soviet-designed reactors up to near western safety standards, or at least to effect significant improvements to the plants and their operation.

The European Union also brought pressure to bear, particularly in countries which aspired to EU membership.

Modifications were made to overcome deficiencies in the 11 RBMK reactors still operating at the time in Russia. State and local governments, with the support of the federal government and the power companies, have emergency plans for nuclear power plant accidents and routinely test their ability to protect people.

In addition, to prevent terrorism, nuclear power plans have special security measures in place to limit access only to authorized people. What are the health effects of a nuclear power plant accident?

How do I prepare for a nuclear power plant accident? Those living within 10 miles of a nuclear power plant should know their designated evacuation zones and routes. Power companies publish this information in telephone books and calendars. For more information, contact your local library or emergency management agency.

If you have a disability and require special assistance for evacuation during an emergency, contact your local emergency management or social services agency to ensure that plans are in place to assist you. Keep a working radio with spare batteries available. Sirens are located in residential areas near nuclear power plants. Know when the sirens will be tested.

Know which radio station to listen to when the sirens sound. In Virginia, people living or working within a 10 mile area of a nuclear power plant will be provided potassium iodide KI for protection from radioactive iodine.

Sustained nuclear fission reactions rely on the passing of neutrons from one atom to another—the neutrons released in one atom's fissioning trigger the fissioning of the next atom. The way to cut off a fission chain reaction, then, is to intercept the neutrons. Nuclear reactors utilize control rods made from elements such as cadmium, boron or hafnium, all of which are efficient neutron absorbers.

When the reactor malfunctions or when operators need to shut off the reactor for any other reason technicians can remotely plunge control rods into the reactor core to soak up neutrons and shut down the nuclear reaction. Can a reactor melt down once the nuclear reaction is stopped?

Even after the control rods have done their job and arrested the fission reaction the fuel rods retain a great deal of heat. What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat.

So the reactor core continues to produce heat in the absence of fissioning. If the rest of the reactor is operating normally, pumps will continue to circulate coolant usually water to carry away the reactor core's heat. In Japan the March 11 earthquake and tsunami caused blackouts that cut off the externally sourced AC power for the reactors' cooling system. According to published reports, backup diesel generators at the power plant failed shortly thereafter, leaving the reactors uncooled and in serious danger of overheating.

Without a steady coolant supply, a hot reactor core will continuously boil off the water surrounding it until the fuel is no longer immersed. If fuel rods remain uncovered, they may begin to melt, and hot, radioactive fuel can pool at the bottom of the vessel containing the reactor. In a worst-case meltdown scenario the puddle of hot fuel could melt through the steel containment vessel and through subsequent barriers meant to contain the nuclear material, exposing massive quantities of radioactivity to the outside world.

How can a meltdown be averted? The Japanese plant's operators have made a number of attempts to cool the reactors, including pumping seawater into the reactor core to replenish the dwindling cooling fluid. Take a look at the beta version of dw. We're not done yet! Your opinion can help us make it better. We use cookies to improve our service for you. You can find more information in our data protection declaration.

After the disasters of an earthquake and tsunami, Japan also suffered under the threat of a meltdown in a nuclear reactor. Here's how a meltdown happens. In a nuclear meltdown, it all comes down to a power plant's reactor.

Japan's Fukushima Daiichi plant, which was damaged by Friday's earthquake, employs six boiling water reactors. When functioning normally, energy production in a boiling water reactor starts with a process known as nuclear fission. That heats up fuel rods in the reactor, which in turn bring water in the reactor to a boil and convert it to steam.

Electricity is produced when the steam passes through a turbine. Then, the water is cooled and pumped back into the reactor and the process can begin again. It's the nuclear fission - how fuel rods are heated up - that is important. During the process, the rods, which are made of enriched uranium, undergo radioactive decay and release large amounts of energy.